DESCRIPTION: The research explores the mechanisms underlying selective visual attention. Thalamic gating of sensory signals is considered to be a fundamental mechanism for regulating attention. The proposed experiments employ complementary anatomical and physiological approaches to examine in the cat the organization of connections between cortex and the pulvinar/lateral posterior (LP) complex, and how these connections are influenced by a projection from cholinergic cells of the of the parabrachial region (PBR). The proposed experiments will test the prediction that shifts in attention are regulated by the dampening or amplification of cortical inputs to the pulvinar and LP nuclei. Anatomical experiments involving cortical injections of the neuroanatomical tracer biocytin will test the hypothesis (#1) that Layer V feedforward and layer VI feedback cortical connections are present in the pulvinar/LP complex. Light and electron microscopic techniques will be used to determine the distribution and postsynaptic targets of type I and type II cortical terminals and will test the prediction that in the pulvinar/LP complex, type II cortical terminals supplant the position occupied by primary sensory afferents in the "specific" thalamic nuclei. In separate experiments in vitro slice preparations, intracellular recordings of pulvinar/LP cells will be obtained to test the hypothesis (#2) that pulvinar/LP cells have distinct cellular responses to selective stimulation of type I and type II cortical terminals. The postsynaptic potentials involved will be characterized (eg. NMDA, non NMDA, metabotropic) and the ability to facilitate responses by high frequency stimulation will be evaluated. Pre and postembedding immunocytochemistry will be used to stain cholinergic terminals and profiles that contain GABA and will test the hypothesis (#3) that PBR terminals in the pulvinar/LP complex are positioned to selectively influence feedforward cortical input. Lastly, ACh will be applied to pulvinar/LP cells in vitro to test (#4) whether ACh enhances the responses of pulvinar/LP cells to cortical input. Collectively, these experiments will lay the groundwork for future physiological and/or behavioral studies of the cholinergic influences on selective attention.